Thickness Estimation Using Conductively Related Calibration Samples

ABSTRACT

A method for monitoring an inspection sample includes generating inspection data comprising resistance and reactance measurements that are obtained from an inspection sample having a conductive layer of unknown thickness. Calibration data is used for estimating the thickness of the conductive layer of the inspection sample. This calibration data includes resistance and reactance measurements obtained from one or more calibration samples, each calibration sample having a conductive layer of known thickness. The conductive layers of the inspection sample and the calibration samples comprise different materials having a known conductive relationship.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/835,975 filed Apr. 17, 2001, which is a continuation-in-part of U.S.application Ser. No. 09/545,119 filed Apr. 7, 2000, now U.S. Pat. No.6,407,546.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an eddy current measuringsystem, and in particular to an eddy current measuring system forestimating the thickness of conductive films formed on semiconductorwafer products.

2. Description of the Related Art

In the semiconductor industry, critical steps in the production ofsemiconductor wafers are the selective formation and removal of films onan underlying substrate. The films are made from a variety ofsubstances, and can be conductive (for example, metal or a magneticferrous conductive material) or non-conductive (for example, aninsulator or a magnetic ferrite insulating material).

Films are used in typical semiconductor processing by: (1) depositing afilm; (2) patterning areas of the film using lithography and etching;(3) depositing material which fills the etched areas; and (4)planarizing the structure by etching or chemical-mechanical polishing(CMP). Films may be formed on a substrate by a variety of well-knownmethods including physical vapor deposition (PVD) by sputtering orevaporation, chemical vapor deposition (CVD), plasma enhanced chemicalvapor deposition (PECVD), and electro-chemical process (ECP). Films maybe removed by any of several well-known methods includingchemical-mechanical polishing (CMP), reactive ion etching (RIE), wetetching, electrochemical etching, vapor etching, and spray etching.

The semiconductor fabrication industry continues to demand higher yieldsand shorter fabrication times, while insisting upon ever-increasingquality standards. A variety of inspection procedures have been employedduring the various stages of the semiconductor wafer fabrication processin an attempt to meet these demands. These inspection procedures includedestructive, as well as nondestructive, testing methods for analyzingwafer products. In a destructive measuring process, a standard orelectron microscope may be used to measure the thickness of a wafer'scoating after a cross-section has been obtained. When the thickness of athin-film coating is greater than 10,000 Å, for example, this type ofdestructive measuring method may provide accurate measurements. However,measuring accuracy usually begins to degrade as the coating thicknessfalls below the 10,000 Å threshold.

Other types of measuring processes utilize sensitive eddy currentsensors which do not destroy or significantly alter the articlemeasured. Although eddy current sensors provide highly accuratereadings, these sensors are susceptible to error. For example, theshifting of an electronic reference point due to thermal drifting oftenoccurs at some point during the data collection and inspection process.To compensate for thermal drifting and to ensure accurate readings, manyexisting eddy current sensors must be recalibrated on a periodic basis.

While there have been other attempts in addition to eddy current sensorsto employ highly accurate, nondestructive measuring devices forestimating the thickness of a conductive top layer formed on asemiconductor wafer product, improvement is still needed.

SUMMARY OF THE INVENTION

A method for estimating a thickness profile of a substrate sample thathas undergone a chemical vapor deposition (CVD) process includesobtaining initial eddy current measurement values while an eddy currentprobe is positioned at an initial distance relative to the substratesample. Terminating values are obtained while the eddy current probe ispositioned at a modified distance relative to the sample. Anintersecting line can be calculated using the initial and terminatingresistance and reactance measurements. An intersecting point between apreviously defined natural intercepting curve and the intersecting linemay also be determined. A reactance voltage of the intersecting pointmay be located along a digital calibration curve to identify aclosest-two of a plurality of calibration samples. The conductive toplayer thickness of the substrate sample can then be determined byapproximating a location, using linear or non-linear calculations, ofthe reactance voltage relative to the closest-two of the plurality ofcalibration samples.

BRIEF DESCRIPTION OF THE DRAWING

The above and other aspects, features and advantages of the presentinvention will become more apparent upon consideration of the followingdescription of preferred embodiments taken in conjunction with theaccompanying drawing, wherein:

FIG. 1 is a diagram showing an eddy current measuring system inaccordance with the invention;

FIG. 2 is a graph showing two-point lift-off curves relating to eddycurrent measurements taken from calibration and substrate sampleshaving, respectively, conductive top layers of known and unknownthicknesses;

FIG. 3 is a graph showing the formation of a natural intercepting curvedefined by initial resistance and reactance values for calibrationsample curves A through E;

FIG. 4 is a graph showing a digital calibration curve that may begenerated by data associated with calibration samples A through E;

FIG. 5 is a flowchart showing exemplary operations for one of a varietyof different methods for estimating the thickness of a conductive toplayer of a substrate;

FIG. 6 is a diagram showing an eddy current measuring system inaccordance with an alternative embodiment of the invention;

FIG. 7 is a side view showing several components of an eddy currentmeasuring system in accordance with the invention;

FIG. 8 provides a bottom view of the eddy current probe support of FIG.7;

FIG. 9 is a top view of the eddy current probe support of FIG. 7positioned over a substrate;

FIG. 10 is a three-dimensional contour map representing a thicknessprofile that may be obtained from a substrate in accordance with theinvention;

FIG. 11 is a graph representing a possible thickness profile that may becreated by obtaining a plurality of thickness measurements over adiameter of a substrate;

FIG. 12 is a block diagram showing an example of an integrated eddycurrent measuring system configured with CVD and CMP systems;

FIG. 13 is a block diagram showing another example of an integrated eddycurrent measuring system of the invention configured with multiple metaldeposition systems;

FIG. 14 is a block diagram showing an example of an integrated eddycurrent measuring system configured with a single metal depositionsystem; and

FIG. 15 is a block diagram showing an example of a standaloneimplementation of an eddy current measuring system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of preferred embodiments, reference is madeto the accompanying drawings, which form a part hereof, and which showby way of illustration, specific embodiments of the invention. It is tobe understood by those of ordinary skill in this technological fieldthat other embodiments may be utilized, and structural, electrical, aswell as procedural changes may be made without departing from the scopeof the present invention.

FIG. 1 is a diagram showing a single-probe eddy current measuring system10 in accordance with some embodiments of the present invention. Asshown, the system includes a conventionally configured eddy currentprobe 12 having sense coil 14, reference coil 16, and capacitance sensor18. The eddy current probe is shown in communication with controller 20,which, during operation, may provide relative motion between the eddycurrent probe and substrate 22. In a typical implementation, thecontroller translates the eddy current probe and included componentsalong vertical axis Z, which is normal to the substrate.

The eddy current probe circuit may be implemented in any suitablemanner. In one particular example, eddy current probe 12 may beconstructed using similarly configured sense and reference coils 14, 16.If desired, the sense and reference coils may each be constructed usingferrite cores, equal coil turns, and similarly sized magnetic cable (forexample, 40 gauge). The circuit may further include AC voltage source 28for inducing an AC voltage to the sense and reference coils, andWheatstone bridge 30. Suitable probes for implementing eddy currentprobe 12 include, for example, absolute pencil probes, model numbersSBS-30 or SB-30, developed by Andrew NDT Engineering, Inc., Morgan Hill,Calif.

In many implementations, AC voltage source 28 may provide pre-selectedsinusoidal waves at a suitable frequency (for example, 1 MHz to 100 MHz,or higher) to the Wheatstone bridge. A sinusoidal wave is often utilizedto maximize phase separation between samples of differing thicknesses,but such a wave pattern is not a required feature.

Sense and reference coils 14 and 16 may be fabricated so that theirrespective inductance values are equal at a given frequency, while theresistance of each coil is less than about 20 Ohms, for example.

In operation, analog signals generated by eddy current probe 12 may befed into an analog to digital (A/D) board 34 which converts the analogsignals into digital signals processed by CPU 36 in accordance with theinvention.

CPU 36 may be configured with a suitable memory unit 38 for storing avariety of different data including, for example, data tables containingcalibration sample data, programmed computer statements which enable acomputer system to act in accordance with the invention, and othersimilar types of data. The memory unit can be any type (or combination)of suitable volatile and non-volatile memory or storage devicesincluding random access memory (RAM), static random access memory(SRAM), electrically erasable programmable read-only memory (EEPROM),erasable programmable read-only memory (EPROM), programmable read-onlymemory (PROM), read-only memory (ROM), magnetic memory, flash memory,magnetic or optical disk, or other similar memory or data storage types.If desired, the system may be configured with display 40.

It is often useful to know or ascertain the relative spatialrelationship between sense coil 14 and a top surface of substrate 22during various stages of operation. To accomplish such measurements, anyof a variety of suitable proximity sensors may be implemented. Asdepicted in FIG. 1, a proximity sensor may be configured as capacitancesensor 18. In this implementation, the capacitance sensor may beconfigured to produce a predetermined voltage in the presence ofinterference or interruption in charge path. In operation, as thecapacitance sensor approaches contact with the substrate, the charge mayexperience interference and produce a voltage drop. A specific ordesired distance may be obtained or maintained by identifying aparticular voltage output generated by the capacitance sensor.

Although the capacitance sensor may be implemented in some embodiments,the invention is not so limited and any of a variety of conventionalproximity sensors may be used including, for example, optical lasers,Hall effect sensors, thermal IR sensors, ultrasound, and the like.Furthermore, it is not required that an implemented proximity sensor beconfigured within eddy current probe 12 and that other configurationsare possible. For example, a proximity sensor may be configured on theoutside of the eddy current probe or on some adjacent structure such asa probe support, as will be described in more detail in the followingfigures. Accordingly, the proximity sensor may be configured in almostany location as long as the relative distance between the sense coil andthe substrate surface can be ascertained or maintained.

Distance 42 represents the relative distance between sense coil 14 andthe surface of substrate 22 where a desired magnitude of eddy currentsignals may be obtained during an initial measuring process. Aparticular implementation may be where distance 42 is about 75 microns,which may be indicated by an output of 5 volts, for example, fromcapacitance sensor 18.

An eddy current measuring system may be implemented in a variety ofapplications where thickness measurements of conductive layers isrequired or desired. Typical applications include, for example,semiconductor fabrication, aerospace industries, metallurgic researchand develop environments, jewelry manufacturing, and the like. As amatter of convenience, various embodiments of an eddy current measuringsystem for performing thickness measurements will be described in thecontext of a typical semiconductor fabrication environment. However, itis to be understood that the invention is not so limited and that manyother applications are envisioned and possible within the teachings ofthis invention.

In a generalized example, substrate 22 may be formed as a semiconductorwafer having a conductive top layer 26. For example, the substrate maybe a doped or an undoped silicon substrate or a substrate upon which oneor more layers of conducting and/or non-conducting underlying films havealready been formed and patterned into gates, wires or interconnects ina multi-level structure. The conductive top layer may be formed usingany of a variety of different deposition processes such aselectrochemical process (ECP), chemical vapor deposition (CVD), physicalvapor deposition (PVD), plasma enhanced CVD (PECVD), low pressure CVD(LPCVD), rapid thermal CVD (RTCVD), atmospheric pressure CVD (APCVD),and the like.

The term “calibration sample” will be used herein to denote a specifictype of substrate, and in particular, a substrate having a conductivetop layer of known thickness. The term “substrate sample” is used hereinto refer to a substrate having a conductive top layer of unknownthickness formed using known semiconductor fabrication processes.

In accordance with many embodiments of the invention, the eddy currentmeasuring system shown in FIG. 1 may be used to measure the thicknessand sheet resistance of a conductive top layer disposed on semiconductorwafer products. To accomplish such measurements, it is typicallynecessary to first measure calibration samples having conductive toplayers of known thicknesses using a calibration process. Typically, anassortment of calibration samples having metal layers of varyingthickness are used during a calibration process. By way of example,calibration samples A, B, C, D, and E will be used herein to define fivesuch calibration samples having a conductive top-layer fabricated with acalibration metal measuring 50,000, 100,000, 150,000, 170,000, and200,000 Å, respectively. The various calibration metals that may be usedinclude, for example, Ti 6-4, Al, Ni, Ni-alloy, stainless steel (300Series), and combinations thereof. Although each of a plurality ofcalibration samples may each include conductive top layers of varyingthicknesses, this is not essential or critical and one or morecalibration samples having a range of top layer thicknesses may be used,if desired.

As will be described in detail herein, calibration measurements obtainedfrom calibration samples may be correlated to eddy current measurementsobtained from a substrate having a conductive top layer of unknownthickness. In some implementations, the invention may be configured toobtain measurements from substrates having conductive top layerscomprising conductive films typically used in the formation ofmulti-level interconnect structures including Cu, Cr, W, Al, Ta, TiN,and combinations thereof.

Usually, the conductive top layers of the calibration and substratesample comprise different types of conductive materials, but this is notrequired. One reason for implementing different conductive materials inthese samples is the aforementioned difficulty in measuring metal layersless than 10,000 Å, for example. As such, many embodiments utilizecalibration samples having top layers of a lower conductivity than thatpresent in a top layer of a substrate sample.

By way of example only, reference will be made to calibration samplescomprising a top layer formed from a relatively lower conductivematerial of annealed Ti 6-4, and substrate samples comprising a toplayer formed from the relatively higher conductive material, annealedcopper. Based upon the well established International Annealed CopperStandard (IACS), the conductivity of annealed copper is the standard bywhich all other electrical conductors are compared. According to thisstandard, the conductivity of annealed copper measures 100 IACS, whilethe lower conductive material of annealed Ti 6-4 is measured as afractional percentage ({fraction (1/100)}) of annealed copper.

Utilizing this known relationship, the conductivity of a particularthickness of annealed copper is equal to a layer of Ti 6-4 that is 100times thicker than the annealed copper. One example of this principal isillustrated by noting that the conductivity of a 1,000 Å layer ofannealed copper is equal to the conductivity of a 100,000 Å layer of Ti6-4, as shown in the following equation:100*1,000Å=100,000Å  Eq. 1

In accordance with some embodiments, measurement of calibration sampleshaving top layers of known thicknesses of Ti 6-4 may be used indetermining the thickness of a copper top layer of a semiconductor waferproduct utilizing the above-described conductive relationship betweenthese materials. And as will be described in detail herein, thecalibration of an eddy current measuring probe for measuring micro-thincopper layers, for example, can be accomplished using a proportionatelythicker layer of material such as Ti 6-4. It is also to be understoodthat the proportional conductive relationship described with respect toTi 6-4 and annealed copper apply to situations involving any of theaforementioned conductive materials that may be used to form thecalibration or substrate samples.

To illustrate the conductive relationship between conductive top layersof calibration and substrate samples of the invention, the following ispresented.ρ=172.41/σ  Eq. 2Where ρ denotes resistivity and σ defines conductivity in IACS units.ρ=Thickness×Sheet resistance=t×R(s)  Eq. 3Where t denotes thickness and R(s) defines sheet resistance, thusproviding the following equation:R(s)=ρ/t  Eq. 4From this relationship, the following equations may be provided:R(s)copper=ρcopper/tcopper  Eq. 5R(s)Ti-6-4=ρTi6-4/tTi6-4  Eq. 6Assume now a calibration sample having a Ti 6-4 top layer that measures100,000 Å, and a measuring sample having a copper top layer that is1,000 Å. Substituting these values into the appropriate above-twoequations provides the following.With regard to the copper top layer of the substrate sample:$\begin{matrix}\begin{matrix}{{{R(s)}\quad{copper}} = {{\left( {171.41/\sigma} \right)/1},000\quad Å}} \\{= {{\left( {{171.41/100}\quad{IACS}} \right)/1},000\quad Å}} \\{= {{171.41/100},000\quad Å\quad{({IACS}).}}}\end{matrix} & {{Eq}.\quad 7}\end{matrix}$With regard to the Ti 6-4 top layer of the calibration sample:$\begin{matrix}\begin{matrix}{{{R(s)}\quad{Ti}\quad 6\text{-}4} = {{\left( {{171.41/1}\quad{IACS}} \right)/100},000\quad Å}} \\{= {{171.41/100},000\quad Å\quad{({IACS}).}}}\end{matrix} & {{Eq}.\quad 8}\end{matrix}$Accordingly, it is demonstrated that:R(s)copper=R(s)Ti6-4  Eq. 9when the Ti 6-4 layer measures 100,000 Å, and the copper layer measures1,000 Å.

Knowledge of electrical behavior, in terms of equivalent sheetresistance, of materials comprising the calibration and substratesamples permit the use of calibration samples having metal layers (Ti6-4) that are 100 times thicker than copper layers formed on substratesamples.

By way of specific example, calibration samples may include conductivetop layers formed from Ti 6-4 having a range of thicknesses such as10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000,or 100,000 Å. Using the previously described conductivity relationshipbetween Ti 6-4 and annealed copper, each of the just-described Ti 6-4layer thicknesses may be used to represent a specific eddy currentresponse of a substrate sample comprising a top layer formed withannealed copper having a thickness of, respectively, 100, 200, 300, 400,500, 600, 700, 800, 900, and 1,000 Å. Accordingly, the measurement of asubstrate sample having a micro-thin copper top layer of unknownthickness disposed upon its surface can be accomplished usingcalibration samples having a correspondingly thicker top layer of Ti 6-4(100 times thicker).

Selection of specific frequency, gain, and voltage drive levels may beused to obtain a maximum magnitude eddy current signal response, whileretaining an ability to determine phase separation at differentthicknesses (for example, 500 Å and 1,000 Å). For a given conductivematerial, such as copper, a calibration sample comprising acorresponding thicker layer of Ti 6-4 may be utilized (as describedabove).

In operation, AC voltage 28 may introduce pre-selected sinusoidal wavesat a suitable frequency to Wheatstone bridge 30. In someimplementations, adjustable electronic bridge circuit 32 may be appliedto the Wheatstone bridge to balance the circuit and zero the referencevoltage. At this point, controller 20 may translate eddy current sensecoil 14 until the coil approaches contact with the surface of substrate22, as indicated by distance 42, at which point the Wheatstone bridgeunbalances its voltage between legs. This voltage may be measured,detecting the amplitude of the in-phase component (X) as well as thequadrature component (Y). As used herein, X voltage values representresistance, while the Y voltage values represent reactance.

FIG. 2 is a graph showing two-point lift-off curves relating to eddycurrent measurements taken from calibration and substrate sample having,respectively, known and unknown thicknesses. This graph will bedescribed with reference to the eddy current measuring system shown inFIG. 1.

As previously described, many embodiments utilize the known conductiverelationship between conductive materials forming the top layers of thecalibration and substrate samples. Based upon this known conductiverelationship, curves representing eddy current measurements obtainedfrom calibration samples comprising top layers formed from variablythick layers of Ti 6-4 may be correlated to substrate samples comprisingtop layers formed from relatively thinner layers of annealed copper.

For example, eddy current measurements obtained from calibration sampleA (comprising a 50,000 Å top layer of Ti 6-4) may be substantiallyidentical to eddy current measurements obtained from a substratecomprising a 500 Å top layer of annealed copper. This relationship holdstrue for each of the remaining curves B through E. For instance, eddycurrent measurements obtained from calibration samples B, C, D, and E(respectively comprising 100,000 Å, 150,000 Å, 170,000 Å and 200,000 Åtop layers of Ti 6-4) are respectively identical to eddy currentmeasurements obtained from samples comprising top layers of annealedcopper measuring 500 Å, 1,000 Å, 1,500 Å, 1,700 Å, and 2,000 Å.

Accordingly, curves representing eddy current measurements obtained fromcalibration samples comprising Ti 6-4 top layers of variable thicknessare identical to curves representing eddy current measurements obtainedfrom samples comprising top layers formed from annealed copper that areof a thickness that is {fraction (1/100)} of that of the Ti 6-4 layers.

Referring still to FIG. 2, curves A through E each include initial datavalues (X,Y) that are illustrated on the left side of this graph, andwhich eventually terminate near the bottom right of this graph (0,0). Ingeneral, the system may obtain two distinct sets of resistance andreactance values, referred to herein as initial and terminatingresistance and reactance measurements. The initial resistance andreactance measurements are typically obtained when the eddy currentsense coil is positioned an initial distance 42 relative to thesubstrate surface.

Terminating resistance and reactance measurements, on the other hand,may be obtained after increasing (or decreasing) the relative distancebetween the eddy current sense coil and the substrate surface. Thisincrease or decrease in distance will be referred to herein as anincremental distance. Initial distance 42 may be any distance thatpermits the detection of sufficiently strong eddy current signals, whilethe incremental distance may be any distance that allows the detectionof initial and terminating measurements representing two discretevalues. In some instances, this may be accomplished by implementing anincremental distance of a few microns, and in other cases, anincremental distance of 20-40 microns, or more, may be required.

A particular example may be where the initial distance is about 75microns, and the incremental distance is about 15-20 microns.Accordingly, in this situation, the terminating resistance and reactancemeasurements may be obtained when the relative distance between the eddycurrent sense coil and the substrate surface is about 90-95 microns.

A possible variation on this aspect may be where terminating resistanceand reactance measurements are obtained after decreasing the relativedistance between the eddy current sense coil and the surface of thesubstrate. In this particular implementation, it is typically necessarythat initial distance 42 is such that the eddy current probe does notcontact the substrate measurements when making the terminatingmeasurements.

A specific example of obtaining eddy current measurements from anassortment of calibration samples comprising variably thick top layersof conductive materials will now be described. Referring still to FIG.2, curve A denotes eddy current measurements that may be obtained fromcalibration sample A comprising a 50,000 Å top layer of Ti 6-4. Curve Ais shown having initial resistance and reactance values (X,Y) of about−0.7 volts and 0.07 volts, respectively. These initial resistance andreactance values may be obtained while eddy current sense coil 14 ispositioned at a particular or desired initial distance 42 relative tothe surface of the calibration sample.

Curve A further includes a series of additional eddy currentmeasurements that ultimately terminate in resistance and reactancevalues (X,Y) near 0,0; thus indicating a measurement of near zeroresistance and reactance voltages. These terminating resistance andreactance voltage values (X,Y) define eddy current measurements obtainedwhen sense coil 14 and the surface of calibration sample A are separatedby such a distance that no eddy current signal is detected. Theseterminating values will also be referred to herein as “eddy current onair.” It is to be understood that terminating values are often obtainedusing eddy current on air values since these types of signals are easilyidentified. However, the invention is not so limited and any incrementaldistance may be used as long as it permits the measuring of initial andterminating values having two discrete values.

Eddy current on air values may be obtained by increasing the relativedistance between sense coil 14 and the surface of the calibration sampleuntil no eddy current signal is detected. Increasing the relativedistance between these elements may be accomplished by retracting eddycurrent probe 12 and included components (sense and reference coils 4and 6; capacitance sensor 18) from the calibration sample. Additionallyor alternatively, the calibration sample may be translated relative tothe eddy current probe.

Curves B through E may be generated in a similar manner. For instance,curves B, C, D, and E illustrate eddy current measurements that may beobtained from calibration samples comprising top layers formed from Ti6-4 of variable thickness (100,000 Å, 150,000 Å, 170,000 Å, and 200,000Å), and are respectively identical to measurements that may be obtainedfrom a substrate sample comprising top layers of annealed coppermeasuring 1,000 Å, 1,500 Å, 1,700 Å, and 2,000 Å.

Initial resistance and reactance values (X,Y) for each curve B through Emay also be obtained by positioning eddy current sense coil 14 atinitial distance 42. Curves B through E also include a series ofadditional measurements that ultimately terminate in resistance andreactance values (X,Y) near 0,0. Curve U is associated with anon-calibration sample, and will be described in more detail inconjunction with later figures.

A calibration operation has been described using five distinctcalibration samples having conductive top layers of variable thickness.Although no particular number calibration samples are required oressential, it is typically necessary to have at least two calibrationsamples of different thicknesses to provide a basis for thicknessestimation. Alternatively, one or more calibration samples providing arange of top layer thicknesses may also be used. Regardless of thecalibration sample configuration, each of the initial resistance andreactance values (X,Y) for each of the curves A through E may be used asthe basis for the generation of a natural intercepting curve, as willnow be described.

Natural Intercepting Curve

FIG. 3 is a graph showing one method for forming a natural interceptingcurve based upon initial resistance and reactance values (X,Y) of curvesA through E.

The natural intercepting curve may be generated using knowncurve-fitting methods, and may be represented in general form by thefollowing equation:Y=me ^(−nX).  Eq. 10The m and n coefficients may be calculated by substituting the initialresistance and reactance values (X, Y) for a particular curve into thisequation.

In another calculation, a linear equation may be generated for aparticular curve based upon two data points; namely, the initialresistance and reactance values (X,Y) and the terminating resistance andreactance values (X,Y). For example, a first data point may be obtainedwhile eddy current sense coil 14 is positioned at initial distance 42relative to the surface of the calibration sample; and a second datapoint may be obtained by increasing (or decreasing) the relativedistance between these two objects.

The first and second data points may then be used to generate a linearequation such as the following:Y=aX+b.  Eq. 11Where ‘a’ defines slope and ‘b’ denotes the offset value present duringdata collection resulting from thermal drift or from measuringdifferences that may occur when different eddy current probes are usedfor measuring the calibration and inspection samples. The collection ofthese two data points is typically less than 1 second, and in somecases, data collection requires less than 0.3 seconds per data point.Coefficients ‘a’ and ‘b’ can be calculated by substituting the value ofthe (X,Y) voltage pair into the equation.

To eliminate the effects of thermal drift and eddy current probemeasuring differences, the ‘b’ coefficient may be eliminated, resultingin the following equation:Y=aX  Eq. 12which will be referred to herein an “intersecting line.” Eliminating the‘b’ coefficient helps assure that the intersecting line is brought backto the original coordinates of the impedance plane (0,0). Intersectingline equations may be generated for each of the calibration samples Athrough E, resulting in calibration sample intersecting lines A, B, C,D, and E, respectively.

Intersection Point

In another calculation, the intersection point of a calibration sampleintersecting line and the natural intercepting curve may be determined.This calculation is performed for each of the calibration samples Athrough E, resulting in calibration sample intersection points A, B, C,D, and E, respectively.

The calculation of these intersection points may be accomplished byequating the natural intercepting curve and one of the above-generatedcalibration sample intersecting line equations, as illustrated in thefollowing equation:me ^(−nX) =aX.  Eq. 13A calculated intersection point may have coordinates (X,Y). The Ycoordinate in the generated intersection point denotes reactance (volts)and will be used as a Y coordinate in forming the digital calibrationthickness curve, as will now be described.

Digital Calibration Curve

FIG. 4 is a graph showing a digital calibration curve generated by dataassociated with calibration samples A through E. In this graph, the Xcoordinate denotes the thickness of the various calibration samples (500Å-2,000 Å) which again have been obtained from the correspondinglythicker layers of Ti 6-4, while the Y coordinate denotes reactance(volts) of the previously generated calibration sample intersectionpoint.

For example, point A represents the top layer thickness and associatedreactance value for calibration sample A. Specifically, point Arepresents a calibration sample A having a copper top layer of about 500Å and a reactance value of about 0.07 volts. Similarly, calibrationsamples B, C, D, and E represent calibration samples having,respectively, copper top layers measuring about 1,000 Å, 1,500 Å, 1,700Å, and 2,000 Å; and associated reactance values of about 0.152, 0.21,0.23, and 0.27 volts.

In many embodiments, a digital calibration curve provides the basis fordetermining the thickness of conductive top layers formed on a givensubstrate such as a semiconductor wafer product. While conventionalsystems require continuous or periodic recalibration to correct thermaldrift, for example, the present invention typically does not require anysuch recalibration. Typically, once the digital calibration curve hasbeen formed, no further calibration processes are necessary.

Measure Thickness of Conductive Top Layer

FIG. 5 is a flowchart showing one of a variety of different methods forestimating the thickness of a conductive top layer of a substratesample, and will be described with reference to the eddy currentmeasuring system shown in FIG. 1, as well as the graphs shown in FIGS.2-4. It is to be understood that at some point prior to thicknessestimation, a digital calibration curve (FIG. 4) has been previouslygenerated by obtaining measurements from one or more calibration samples(described above).

As indicated in block 50, eddy current sense coil 14 may be initiallypositioned at initial distance 42 relative to the surface of substratesample U. At this point, initial resistance and reactance values (X,Y)of the substrate sample U may be obtained.

Next, the relative distance between the sense coil and the substratesample U may be increased (or decreased) an incremental distance so thatterminating resistance and reactance voltage values (X,Y) may beobtained (Block 55). Curve U in FIG. 3 provides an example of initialand terminating resistance and reactance values (X,Y) for the substratesample U.

In block 60, the initial and terminating resistance and reactance values(X,Y) of the substrate sample U may be used in the following equation:Y=aX+b.  Eq. 14In many instances, the ‘b’ coefficient may be eliminated to correct forthermal drift and eddy current probe measuring differences, resulting inthe following equation:Y=aX.  Eq. 15This equation is referred to herein as intersecting line U.

As shown in block 65, the intersection point between the naturalintercepting curve and the wafer substrate intersecting line U may bedetermined by the following equation:me^(−nX) =aX.  Eq. 16

The generated intersection point may have a coordinate of (X,Y).Significantly, the Y value in the generated intersection pointcoordinates denotes reactance (volts) of the intersection point. Inblock 70, this Y value is located along the Y axis of the previouslygenerated digital calibration curve (FIG. 4) so that the closest-twocalibration samples of known thickness may be determined or ascertained.

For example, the Y coordinate associated with the substrate sample Uwill typically fall within the Y coordinates of two distinct calibrationsamples. As shown in FIG. 4, the Y coordinate of the generated substratesample intersection point falls between two calibration samples (A andB), thus indicating that the top layer thickness of the substrate sampleU measures between 500 Å and 1,000 Å.

As indicated in block 75, the top layer thickness of the substratesample U may be more precisely determined by performing linear ornon-linear calculations. For example, an appropriate linear calculationmay be accomplished by performing an interpolation between theappropriate two calibration samples (for example, calibration samples Aand B). On the other hand, a non-linear calculation may be implementedby curve-fitting the Y coordinate associated with the substrate sample Uto the curve defined by the appropriate two calibrations samples.

FIG. 6 is a diagram showing an eddy current measuring system accordingto an alternative embodiment of the present invention. In this figure,AC voltage source 28 is in electrical communication with sense coil 14,reference coil 16, as well as capacitance probe 18. In this embodiment,reactance and resistance may be detected by reactance detector 80 andresistance detector 82, respectively. A detected signal may be amplifiedutilizing an automatic gain control circuit, denoted by referencenumbers 84, 86, 88, 90, and 92. Vector rotation 94 may be utilized torotate the signal so that an appropriate graphical presentation may bepresented at optional display 40. CPU 36 and memory 38 may operate inthe same manner as the eddy current measuring system shown in FIG. 1.

FIG. 7 is a side view showing several components of an eddy currentmeasuring system in accordance with the invention. In this particularembodiment, controller 20 is coupled with eddy current probe support 150containing a linear array of individual eddy current probes 12. FIG. 8provides a bottom view of the eddy current support and associated arrayof individual eddy current probes.

In operation, substrate 22 may be securely positioned using a suitablewafer securing device such as a conventional wafer chuck 155. Typically,the chuck includes a vacuum or other suitable securing device forstabilizing the substrate during the thickness measuring process.

Implementing an array of multiple eddy current probes is particularlyuseful for simultaneous inspection or monitoring of thicknesses ofmultiple locations of the substrate. Although seven separate eddycurrent probes are shown, additional or fewer probes may be implementedas may be desired or required. For example, it is contemplated that thenumber of eddy current probes may range anywhere from a single probe, toas many as 25-30 probes, or more. Other variations may be to stagger oroffset an array of multiple probes along the bottom side of probesupport 150 in non-linear fashion. In addition, the use of multipleprobe supports containing one or more eddy current probes may also beused. Examples of various multiple probe support embodiments that may beimplemented include arranging the probe supports in parallel fashion, orat some angular orientation relative to one another.

FIG. 9 is a top view of eddy current probe support 12 positioned oversubstrate 22. Thickness measurements of the substrate sample may beaccomplished by providing relative motion between the substrate and eddycurrent probes 12 so that initial and terminating measurements may beobtained. One particular example may be where the probe support andassociated array of eddy current probes is rotated in direction 157.

Another example may be where the substrate is rotated relative to theeddy current probes. This may be accomplished by removing the substratefrom its position on chuck 155, rotating the substrate a desired numberof degrees, and repositioning the substrate on the chuck. Alternatively,the chuck may be configured with a suitable control device for rotatingthe substrate sample. Yet another variation may be where the eddycurrent probe support and the substrate sample are rotated relative toeach other.

Linear measuring methods may also be implemented to supplement, or asalternative, to the just-described rotational measuring options. Forinstance, thickness measurements of a substrate may be accomplished bylinearly translating the substrate, probe support, or both, in the X orY direction.

FIG. 10 is a three-dimensional contour map representing a thicknessprofile that may be obtained from a substrate sample in accordance withthe invention. In this figure, substrate sample 22 includes variouselevations that may be associated with a particular thickness. Datanecessary for generating the contour map may be obtained by scanning thesubstrate sample with one or more eddy current probes, and making theappropriate thickness measurement at discrete locations on the substratesample using any of the methods described herein. For example, thesubstrate sample may be scanned in radial fashion starting at or nearthe center and progressing in an outward manner, or vice versa.Alternatively, an eddy current probe (or probes) may be raster scannedover the desired locations of the substrate.

Other possibilities include the use of multiple eddy current probesconfigured on an eddy current probe support 150, as depicted in FIGS.7-9. Scanning the substrate sample to obtain a number of thicknessmeasurements may be implemented using a rotational scanning method, alinear scanning method, or both.

Initially, regardless of the type of scanning method employed, the probesupport containing a multiple array of eddy current probes may bepositioned over a starting location of the substrate so that thicknessmeasurements of these particular locations may be obtained.

In another operation, if a rotational method is utilized, the eddycurrent probe support, the substrate, or both, may be rotated apre-determined number of degrees relative to one another. Upon doing so,the newly positioned eddy current probes may obtain thicknessmeasurements of different locations of the substrate. These proceduresmay be repeated until all of the required thickness measurements havebeen made.

A particular example of a rotational scanning method may be where theprobe support is rotated 10° relative to an underlying substrate sample.Upon the performance of this rotation operation, the array of eddycurrent probes can obtain measurements from different locations of thesubstrate. If this rotation operation is repeated seventeen times (10°increments) the eddy current probes will have been rotated a total of170°, thus providing a complete scan of the substrate.

Of course, the incremental degree of rotation may be varied toaccommodate a particular measuring requirement. For instance, a total of179 rotations and corresponding measurements where a rotationalincrement of 1° is used may provide for an extremely accurate thicknessprofile of the substrate sample. On the other hand, where such a degreeof accuracy is not essential, a single rotation of 90° and correspondingmeasurement may provide sufficient thickness data.

Using a linear translation method, the eddy current probe support andassociated probes may be translated along the X and/or Y axes relativeto the substrate. Upon doing so, the newly positioned eddy currentprobes may obtain thickness measurements of different locations of thesample. These procedures may be repeated until all of the requiredthickness measurements have been obtained.

FIG. 11 is a graph representing a possible thickness profile that may becreated by obtaining a plurality of thickness measurements over adiameter (or other portion) of a substrate sample in accordance with theinvention. In this graph, the bottom axis represents the diameter ofsubstrate 22, while the radial center of the substrate is provided indashed lines. The vertical axis denotes some of the possible top layerthickness of a substrate sample. Measurement data depicted in FIG. 11may be obtained using any of the thickness measurement techniquesdescribed herein.

The left side of the graph indicates that the leftmost edge of asubstrate sample has a thickness of about 750 Å. The thickness of thesample continues to rise to about 1,500 Å, where it then declines toabout 1,250 Å at about the radial center of the sample. The thickness ofthe substrate again rises and then falls to about 1,000 Å near the rightside of the substrate.

Knowledge of the top layer thickness of a substrate, which may beobtained using any of the various methods described herein, is useful ina range of applications. By way of example, an assortment of the manypossible implementations of an eddy current measuring system will now bedescribed.

In general, the illustrated embodiments may be characterized as anintegrated or standalone eddy current measurement system (ECMS). Anintegrated ECMS may include a system that is integrated or tightlycoupled with metal deposition, or metal layer removal systems present inconventional semiconductor wafer fabrication environments. Examples ofthe possible metal deposition systems and processes that may implementan ECMS include ECP, CVD, PVD, PECVD, LPCVD, RTCVD, APCVD systems, amongothers. A chemical mechanical polishing (CMP) system is one example of ametal layer removal system that may implement an integrated ECMS.

An example of an integrated ECMS may be where the ECMS is physicallyseparated from an associated ECP system, for example, but were the ECMScommunicates or provides thickness data to the ECP and/or a CMP systemusing some type of communication link (for example, UTP, networkcabling, coaxial cables, serial or parallel cables, fiber optics,wireless link, among others). Another example, of an integrated systemmay be where the ECMS is physically separated from the ECP system, butthe ECP system presents substrates to the ECMS using some type ofmechanical device such as a robot or conveyor. Physically configuringsome or all of an ECMS system with an ECP system may also constitute anintegrated system.

A standalone ECMS system may be characterized as an ECMS that is notcoupled in some manner to a particular metal deposition or metal layerremoval system. In essence, a standalone system is a system that doesnot meet the requirements of an integrated system. In someimplementations, as will be described in detail herein, a standalonesystem may operate as a functional tool within a complete semiconductorfabrication environment. In other embodiments, a standalone system maybe employed to obtain thickness measurement data so that one or morediscrete components of a semiconductor fabrication system may bemonitored or controlled. Other applications include implementing thestandalone ECMS as a table-top device, which has particular appeal tothose working in research and development environments.

It is to be understood that the various ECMS systems depicted in thefollowing figures may be may be fabricated using any of the eddy currentmeasuring systems and methods disclosed herein. Furthermore, the variousmetal deposition and metal layer removal systems depicted in FIGS. 12-15may be implemented using conventional semiconductor fabrication systemcomponents, but with modified tooling to accommodate and utilize anassociated ECMS system.

For example, the various metal deposition systems, CVD 210, PVD 255, andECP 260, may be implemented using any suitable system providingdeposition of thin metallic films using standard and well knowndeposition processes. In general, a metal deposition process involvesdepositing a filler layer over a non-planar surface of a wafer, which isone particular example of a substrate sample. For example, a conductivefiller layer may be deposited on a patterned insulated layer to fill thetrenches or holes in the insulated layer.

Similarly, CMP 220 may be implemented using any suitable systemproviding metal layer removal, a specific example of which is achemical-mechanical polishing (CMP) system. A wafer typically undergoesprocessing by a CMP system after a metal deposition process. The CMPsystem typically polishes the conductive layer until the raised patternof the insulated layer is exposed. After planarization, the portions ofthe conductive layer remaining between the raised pattern of theinsulated layer form vias, plugs and lines that provide conductive pathsbetween thin film circuits on the substrate.

The CMP process typically requires the mounting of the wafer on acarrier or polishing head. The exposed surface of the wafer may beplaced against a rotating polishing disk pad or belt pad. The polishingpad can be either a “standard” pad or a fixed-abrasive pad. A standardpad has a durable roughened surface, whereas a fixed-abrasive pad hasabrasive particles held in a containment media. The carrier headtypically provides a controllable load on the substrate to push itagainst the polishing pad. Polishing slurry, including at least onechemically-reactive agent, and abrasive particles if a standard pad isused, is supplied to the surface of the polishing pad.

One problem in CMP is determining whether the polishing process iscomplete; that is, whether the top layer of the wafer has beenplanarized to a desired flatness or thickness, or when a desired amountof material has been removed. Overpolishing (removing too much) of aconductive top layer or film may lead to increased circuit resistance.On the other hand, under-polishing (removing too little) of a conductivetop layer may lead to electrical shorting. Variations in the initialthickness of the top layer of a wafer may cause variations in thematerial removal rate. Accordingly, knowledge of top layer thickness ofa wafer is particularly useful in the many systems involved in thesemiconductor fabrication process.

FIG. 12 is a block diagram showing an example of an integrated ECMSconfigured with CVD and CMP systems. In this figure, an ECMS 205 isintegrated with CVD system 210, while ECMS 215 is configured with CMPsystem 220. In this arrangement, each ECMS system may communicate orprovide thickness data in two distinct manners referred to herein asfeed forward and feed back operations.

The CVD and CMP systems depicted in this figure represent aconventionally configured semiconductor fabrication setup, as modifiedto accommodate and utilize an associated ECMS system. A typicalsemiconductor fabrication process utilizing an ECMS system may proceedas follows.

First, the CVD system may process a batch of wafers resulting in thedeposition of a top layer comprising, in many instances, copper or otherconductive material. One or more of the processed batch of wafers may bepresented to ECMS 205 for thickness measurements. Next, ECMS 205 mayperform the required thickness measurements of the selected wafer orwafers, which typically takes a few seconds per wafer, and then providesthe generated thickness data to the CVD system using a feed backoperation. In this configuration, ECMS 205 provides near real-timeprocess control or monitoring of the CVD metal deposition process. TheCVD system may use this thickness data so that it can adjust its processparameters for processing subsequent batches of wafers. Typical processparameters for a CVD system include process time, current or voltagevalues, solution density, and the like.

As an alternative, or in addition to providing the feed back operation,ECMS 205 may also provide thickness data to CMP system 220 in a feedforward operation. The CMP system may use the thickness data so that itcan optimize the metal removal process of the processed batch of wafers.For example, after a batch of wafers have been processed by the CVDsystem, they may be transported (human operator, robotics, etc.) to theCMP system so that a portion of the just-deposited top layer may beremoved using, for example, a CMP planarization process.

Notably, any of the ECMS systems provided herein can provide thicknessmeasurements of patterned and un-patterned wafers and is therefore notreliant upon the use of non-yielding measuring blanks. Implementing anECMS system may therefore permit an increase in overall wafer yieldssince the measuring blanks may be replaced with useable patternedwafers. Measurement accuracy is another benefit that may be provided byan ECMS system since the actual patterned wafers, not measuring blanks,undergo thickness measurements.

Typically, one or several of the many wafers of a processed batch ofwafers are actually measured by the ECMS systems during the fabricationprocess. However, every wafer of a process batch may each beindividually measured, if desired.

Similar to the CVD system, the CMP system may also use the thicknessdata to adjust its process parameters to provide optimal processing. CMPprocess parameters may include the relative positioning of a polishingpad on the wafer, pad velocity, pad pressure, polishing time, slurryrecipe, and the like.

If desired, the CMP system may present one or more of the planarizedbatch of wafers to ECMS 215 so that post-CMP thickness measurements maybe obtained. In this scenario, ECMS 215 may perform the requiredthickness measurements of the selected wafer or wafers and then providethe generated thickness data to the CMP system in a feed back operation.In this configuration, ECMS 215 provides near real-time process controlor monitoring of the CMP process. Alternatively or additionally, ECMS215 may also communicate or provide the generated post-CMP thicknessmeasurement data to the CVD system in a feed forward operation.

Any of the ECMS systems described herein may perform thicknessmeasurements on processed wafers on a periodic or continuous basis asmay be required or desired in a particular application. For example, insome instances, one or more wafers of every batch of wafers may bemeasured by the ECMS system. In other situations, it may be optimal tomeasure processed wafers on a predetermined or random basis (forexample, every hour, once a day, once a week, etc.)

Suitable systems for implementing CVD 210 include, for example, theEndura Electra Cu CVD system marketed by Applied Materials, Inc., ofSanta Clara, Calif., and the Altus line of CVD systems developed byNovellus Systems, Inc., of San Jose, Calif. An example of a suitable CMPsystem 220 includes the Reflexion CMP system of Applied Materials, andMomentum 200 or 300 CMP systems marketed by Novellus Systems.

FIG. 13 is a block diagram showing multiple metal deposition systemsconfigured with integrated ECMS systems. In this figure, ECMS systems255, 260, and 205 are shown respectively integrated with PVD system 265,ECP system 270, and CVD system 210, while ECMS 215 is configured withCMP system 220. In this arrangement, thickness data of processed wafersmay be obtained from any of a variety of different metal depositionsystems. This particular embodiment may be implemented to provide systemcontrol or monitoring during semiconductor fabrication where differentdeposition processes (for example, PVD, ECP, CVD, etc.) are utilizedduring particular stages of fabrication.

Similar to other embodiments, each of the metal deposition and removalsystems depicted in this figure represent a conventionally configuredsemiconductor fabrication setup, as modified to accommodate and utilizean associated ECMS system. A typical semiconductor fabrication processutilizing an ECMS system within a multiple metal deposition systemenvironment may proceed as follows.

First, the PVD system may process a batch of wafers according to wellknown PVD processing methods, resulting in the deposition of conductivematerial on a wafer substrate. The PVD system may then present one ormore of the processed batch of wafers to ECMS 255 for thicknessmeasurements. After making the required thickness measurements, ECMS 255may communicate the generated thickness data to the PVD system in a feedback operation, and to CMP 220 system in a feed forward operation.

As before, the CMP system may also use the thickness data to adjust itsprocess parameters to provide optimal processing of the batch of wafers.After processing, the CMP system may present one or more of theplanarized batch of wafers to ECMS 215 so that post-CMP thicknessmeasurement may be obtained. After performing the required thicknessmeasurements, ECMS 215 may communicate the generated thickness data tothe CMP system in a feed back operation. Alternatively or additionally,ECMS 215 may also communicate or provide the generated post-CMPthickness measurement data to some or all of the metal depositionsystems in a feed back operation.

At some point, wafer processing may proceed by subjecting the batch ofwafers to additional deposition processes using, for example, any of thedeposition systems depicted in FIG. 13. In some cases the wafers mayundergo repeated layering cycles using the same metal deposition process(for example, repeated PVD metal deposition cycles). In othersituations, the wafers may be further processed using alternating orvarying metal deposition cycles (for example, PVD→CVD→CVD→ECP→PVD→CVD,etc.). Regardless of the particular deposition processes utilized(repeated or varying), an integrated ECMS system configured with aparticular deposition system may perform the required thicknessmeasurements and provide the generated thickness data using theappropriate feed back and feed forward operations previously discussed.

The PVD and ECP systems may use the calculated thickness data so thatthey can adjust their respective process parameters for processingsubsequent batches of wafers. Typical process parameters for the PVD andECP systems include one or more parameters such as process time, currentor voltage values, solution density, ion source, chamber temperature,and the like.

Examples of a suitable PVD system 265 include the INOVA line of PVDsystems marketed by Novellus Systems. The Electra Cu system developed byApplied Materials is one example of an ECP system that may be used forimplementing ECP system 270.

FIG. 14 is a block diagram showing an example of an integrated ECMSconfigured with a single metal deposition system. In this figure, afully functional ECMS system 205 is shown integrated with CVD system210. This arrangement is often implemented whenever the control ormonitoring of a discrete semiconductor fabrication processes is desired.In this specific example, ECMS system 205 is used for generatingthickness measurements of wafers processed by CVD system 210. A similararrangement may be employed for control or monitoring of any of theother metal deposition systems.

It is to be understood that each deposition system can be configuredwith a fully functional ECMS system (feed forward and feed backcapabilities), or with an ECMS system that provides either a feedforward or a feed back operation. Other possibilities includeimplementing an ECMS system in a limited number of semiconductorfabrication systems. A particular example may include configuring CMPsystem 220 with a fully functional ECMS system 215, while none of theother metal deposition systems implemented have an ECMS system. Anotherexample is where all of the metal deposition systems have an ECMSsystem, but CMP system 220 does not have an ECMS system.

FIG. 15 is a block diagram showing an example of a standaloneimplementation of an ECMS. In this figure, standalone ECMS system 325 isshown in a relative spatial relationship to PVD system 255, ECP system270, CVD system 210, and CMP system 220. This implementation is similarto the configuration depicted in FIG. 13. However, a notable distinctionis that the configuration of FIG. 15 does not include the communicationof thickness data to any of the various other systems using, forexample, feed back and feed forward operations. ECMS system 325functions as standalone unit.

This particular embodiment may be implemented to provide systemmonitoring during semiconductor fabrication where different depositionprocesses (for example, PVD, ECP, CVD, etc.) are utilized during variousstages of fabrication. A particular example may be where a systemoperator may remove particular wafers from the various fabricationsystems and manually or mechanically present the wafers to ECMS 325 forthickness measurements. ECMS 325 may then perform the required thicknessmeasurements and present the generated thickness data to the systemoperator using a display screen, printer, or other suitable outputdevice. If desired, the operator may use this thickness data to verifythe performance of each of these fabrication systems.

While the invention has been described in detail with reference todisclosed embodiments, various modifications within the scope and spiritof the invention will be apparent to those of ordinary skill in thistechnological field. It is to be appreciated that features describedwith respect to one embodiment typically may be applied to otherembodiments. Therefore, the invention properly is to be construed withreference to the claims.

1-28. (canceled)
 29. A method for monitoring an inspection sample,comprising: generating inspection data comprising resistance andreactance measurements obtained from said inspection sample having aconductive layer of unknown thickness; and estimating thickness of saidconductive layer of said inspection sample using calibration datacomprising resistance and reactance measurements obtained from one ormore calibration samples, each having a conductive layer of knownthickness, wherein said conductive layers of said inspection sample andsaid one or more calibration samples comprise different materials havinga known conductive relationship.
 30. The method according to claim 29,further comprising: providing said estimated thickness of saidconductive layer of said inspection sample to a metal layer removalsystem.
 31. The method according to claim 29, further comprising:providing said estimated thickness of said conductive layer of saidinspection sample to a metal deposition system.
 32. The method accordingto claim 29, further comprising: identifying a polishing processendpoint using said estimated thickness of said conductive layer of saidinspection sample.
 33. The method according to claim 29, wherein theconductivity of said conductive layer of said one or more calibrationsamples is lower than the conductivity of said conductive layer of saidinspection sample.
 34. The method according to claim 29, wherein saidconductive layer of said inspection sample is of a different thicknessthan said conductive layer of said one or more calibration samples. 35.The method according to claim 29, wherein said conductive layer of saidinspection sample is thinner than said conductive layer of said one ormore calibration samples.
 36. The method according to claim 29, furthercomprising: adjusting process parameters in which said inspection sampleis processed using said estimated thickness of said conductive layer ofsaid inspection sample.
 37. The method according to claim 29, furthercomprising: providing said estimated thickness of said conductive layerof said inspection sample to a metal layer removal system; andidentifying a polishing process endpoint using said estimated thicknessof said conductive layer of said inspection sample.
 38. Acomputer-readable medium containing instructions for controlling acomputer system to monitor an inspection sample according to a methodcomprising: generating inspection data comprising resistance andreactance measurements obtained from said inspection sample having aconductive layer of unknown thickness; and estimating thickness of saidconductive layer of said inspection sample using calibration datacomprising resistance and reactance measurements obtained from one ormore calibration samples, each having a conductive layer of knownthickness, wherein said conductive layers of said inspection sample andsaid one or more calibration samples comprise different materials havinga known conductive relationship.
 39. The computer-readable mediumaccording to claim 38, said method further comprising: providing saidestimated thickness of said conductive layer of said inspection sampleto a metal layer removal system.
 40. The computer-readable mediumaccording to claim 38, said method further comprising: providing saidestimated thickness of said conductive layer of said inspection sampleto a metal deposition system.
 41. The computer-readable medium accordingto claim 38, said method further comprising: identifying a polishingprocess endpoint using said estimated thickness of said conductive layerof said inspection sample.
 42. The computer-readable medium according toclaim 38, said method further comprising: adjusting process parametersin which said inspection sample is processed using said estimatedthickness of said conductive layer of said inspection sample.
 43. Asystem for monitoring an inspection sample, comprising: means forgenerating inspection data comprising resistance and reactancemeasurements from said inspection sample having a conductive layer ofunknown thickness; and means for estimating thickness of said conductivelayer of said inspection sample using calibration data comprisingresistance and reactance measurements obtained from one or morecalibration samples, each having a conductive layer of known thickness,wherein said conductive layers of said inspection sample and said one ormore calibration samples comprise different materials having a knownconductive relationship.